The invention relates to a method for the internal cleaning of a tank, in particular a crude-oil tank, an oil sump and a gas mixture which is diffused out of the oil sump and has a hydrocarbon-containing gas being present in the crude-oil tank.
The invention relates, furthermore, to a device provided for this purpose and to a cleaning vehicle provided for this purpose.
A method and a device for the internal cleaning of a tank, in particular of a crude-oil tank, are generally known.
Crude-oil tanks are used for the storage of relatively large quantities of crude oil with a capacity of approximately 100 000 m3. Crude-oil tanks of this type may be designed, for example, as floating-top tanks, in which a disc-shaped roof floats on the crude oil and can move up and down in a vertical direction when crude oil flows in or flows out.
Over the years, deposits form on the bottom of crude-oil tanks of this type, which occur as a result of the sinking of heavy constituents of the crude oil and are designated as gatch or crude-oil sump. The deposits often have a sediment-like solid consistency and a comparatively greasy surface. The type and thickness of the deposits depend, inter alia, on the grade of crude oil stored, on the storage quantity, on the transfer frequency and on the tank size. The deposits have to be removed at regular intervals, for which purpose, as a rule, the crude oil is first discharged and then the deposits are removed from the tank bottom mechanically.
After the crude oil has been discharged, a gas and/or vapour mixture occurs above the crude-oil sump. The gas and/or vapour mixture has gases which are diffused out of the crude-oil sump and, as a rule, comprise volatile hydrocarbon compounds. The content of the hydrocarbon-containing gas and/or vapour mixture in the interior of the tank may amount to 30 000 ppm after the discharge of the crude oil. Before the removal work can be carried out, the gas and/or vapour mixture which has occurred above the crude-oil sump has to be sucked away. This is necessary, since the cleaning work is normally performed by cleaning personnel and, for reasons of work safety, the gases harmful to health must be removed from the interior of the tank. Before personnel can enter the tank for cleaning purposes, the concentration of the hydrocarbons in the interior of the tank must be lower than 2000 ppm, as prescribed in work safety regulations.
Even when cleaning vehicles are used, which, as a rule, are robot-like crawler vehicles, the presence of a gas mixture is not desirable, since this gas mixture is, as a rule, ignitable when it comes into contact with oxygen.